Synthesis and Release of Polypeptides by Pig Conceptuses During the Period of Blastocyst Elongation and Attachment12

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Abstract
Pig blastocysts, cultured in a modified minimal essential medium in the presence of radioactive L-leucine, released radiolabeled macromolecules into the medium at a linear rate for at least 24 h. Proteins released into the medium by blastocysts during the period of expansion through attachment were identified by 2-dimensional polyacrylamide gel electrophoresis and fluorography. Spherical (4-9 mm), tubular (10-50 mm) and early filamentous blastocysts (> 100 mm length), recovered between days 10.5-12 of pregnancy, all released, as their major products, a group of low MW (Mr) [relative MW] acidic polypeptides (Mr 20,000-25,000; pI 5.6-6.2). By day 13 the pattern of protein synthesis changed markedly, and the major proteins detected between days 13-16 were basic and in the Mr range of 35,000-50,000. A high MW glycoprotein of > 650,000 was also released. The major basic polypeptide (Mr .simeq. 45,000) and the high MW glycoprotein were isolated from day 16 blastocyst incubations. After day 18, a distinctive group of new polypeptides in the 50,000-70,000 MW range was synthesized. These correspond in electrophoretic mobilities to fetal serum proteins, including transferrin, .alpha.-fetoprotein and fetuin, and appear to be primarily products of the embryo and/or yolk sac. Explant cultures of day-25 and -30 chorioallantois cultured without embryonic tissue released 3 major polypeptides into the medium which were distinct from any synthesized during the preattachment period (days 10.5-16). Although little is known about the physiological role of conceptus proteins produced early in pregnancy, the identification and isolation of these products is a necessarly 1st step in determining their function.